In the technical sector in question, in particular in the field of cooking ovens for foods, apparatuses are widely used which are equipped with fans acting within the cooking chambers and capable of providing forced air circulation, necessary for adequately ensuring the process of cooking the food. Typical applications relate to the use of fans driven by electric motors. In this field there is a need to be able to vary the rotation speed of the fan in order to control the temperature inside the cooking chamber, according to the cooking cycles required. One type of known regulating system provides for the use of inverter devices, which however prove to be particularly troublesome in the range of apparatuses to which the invention is addressed. Other known types provide for “wave-cut” systems or “multi-winding” motors, these systems also proving rather expensive in relation to the functional character required in the respective applications. Moreover, such systems often have characteristics and degrees of accuracy in the regulation of the fan speed which are rather high, and which prove unjustified in relation to the temperature gradients which are in any case acceptable in the cooking cycles, and having much wider tolerances than those ensured by the aforesaid systems. In addition, the aforesaid regulating devices in any case involve dispersions of thermal energy, more or less significant, which however have a negative effect on the thermal balance of the cooking apparatus.
Another known system provides for the use of electric motors in which the speed variation is obtained by connecting one or more electrical resistors in series with the motor. By means of the electrical supply of such resistors, corresponding reductions in the rotation speed of the motor are obtained. These systems however, although proving to be simpler than those previously mentioned, involve the limitation of the dispersion of thermal energy, produced in the electrical resistors as the current passes through, with a consequent lower overall output of the apparatus.